1. Technical Field of the Invention
The present invention relates to integrated low dropout linear voltage regulators and, in particular, to low dropout linear voltage regulators providing improved current limiting.
2. Description of Related Art
Linear voltage regulators are widely used in the power supply circuits of electronic designs. In many applications these regulators are required to operate with small input-output voltage differentials. Low dropout (LDO) linear voltage regulators are a class of linear voltage regulators that are specifically designed to provide this capability. Linear voltage regulators, including LDOs, also normally incorporate special circuitry for protecting both the load and the regulator under abnormal conditions such as “overload.” The most common protection mechanism used is “current limiting.” The vast majority of integrated linear voltage regulators (linear voltage regulators implemented in the form of monolithic integrated circuits) incorporate such protection mechanisms.
For implementing current limiting, the regulator circuit includes an arrangement to sense the current conducted by the output transistor and limit that current to a predetermined safe maximum value when overload occurs, such as an output short-circuit.
The most common method to provide current limiting is by providing a resistor in series with the output and sensing the voltage drop. The voltage drop across the resistor, which is proportional to the output current of regulator, is compared with a preset voltage. The drive to the output transistor is then limited or cutoff if the sensed voltage exceeds the predefined voltage.
U.S. Pat. No. 4,851,953 describes a low drop out voltage regulator based on this principle. According to this invention a series resistor is inserted in the output current path to sense the output current as shown in FIG. 1. The voltage drop across this sense resistance is proportional to the output current of regulator and is fed back to a current limit circuit which controls the drive of the output transistor to limit the current. This arrangement suffers from the drawback that the sense resistor causes a voltage drop leading to an undesired increase in voltage dropout.
U.S. Pat. No. 4,254,372 describes a current sensing method for Low Dropout regulators. In this method, instead of inserting a resistor in the output path, a sense resistance is inserted in the path of the base current drive of the PNP series pass transistor. The base current is sensed through the sense resistance and is used to control the output current by limiting the base current to a predetermined value corresponding to a maximum allowable load current. However, this arrangement can only be used when the output pass transistor is a Bipolar Junction Transistor (BJT). Modern integrated circuits based on Metal Oxide Semiconductor (MOS) transistors cannot therefore utilize this technique. The technique is also not very convenient for BJT applications owing to the wide variation in current gain between individual series pass transistors.